This invention relates to a respiratory instrument for measuring metabolism and in particular to a metabolic calorimeter for relating respiratory parameters by indirect calorimetry.
U.S. Pat. Nos. 5,038,792; 5,178,155; 5,179,958; and 5,836,300, all to the same inventor as the present application, disclose systems for measuring metabolism and related respiratory parameters through indirect calorimetry. These instruments employ bi-directional flow meters which pass both the inhalations and the exhalations of a user breathing through the instrument and integrate the resulting instantaneous flow signals to determine total full flow volumes. The concentration of carbon dioxide generated by the user is determined by either passing the exhaled volume through a carbon dioxide scrubber before it passes through the flow meter, so that the differences between the inhaled and exhaled volumes is essentially a measurement of the carbon dioxide contributed by the lungs, or by the measurement of the instantaneous carbon dioxide content of the exhaled volume with a capnometer and integrating that signal with the exhaled flow volume. The oxygen consumption can then be calculated.
The scrubber used with certain of these systems was relatively bulky and required replenishment after extended usage. The capnometers used with the instruments to measure carbon dioxide concentration had to be highly precise and accordingly expensive because any error in measurement of the carbon dioxide content of the exhalation produces a substantially higher error in the resulting determination of the oxygen contents of the exhalation. Thus, there is a need in the art for an indirect calorimeter for measuring the metabolic activity and related respiratory parameters of a user.
The present invention is an indirect calorimeter for measuring the metabolic activity of a subject. The indirect calorimeter includes a respiratory connector operative to be supported in contact with the subject so as to pass inhaled and exhaled gases therethrough as the subject breathes, and a flow tube forming a flow pathway for passing inhaled and exhaled gases therethrough, wherein one end of the flow tube is operatively connected to the respiratory connector and the other end of the flow tube is open, and a wall of the flow tube includes an opening. The indirect calorimeter also includes a flow meter adapted to generate a signal as a function of the instantaneous volume of inhaled and exhaled gases in the flow pathway that is in fluid communication with the flow pathway via the opening in the flow tube, and an oxygen sensor operative to generate a signal as a function of the instantaneous fraction of oxygen in the inhaled and exhaled gases in the flow pathway that is in fluid communication with the flow pathway via the opening in the flow tube. The indirect calorimeter further includes a processor for receiving the signals from the flow senor and the oxygen sensor and using the signals to determine the oxygen consumption of the subject over a period of time.
The present invention overcomes the disadvantages associated with prior art indirect calorimeters by providing a respiratory calorimeter in which both the inhaled and exhaled flow volumes pass through a flow meter which provides an output representative of the instantaneous flow rate and the inhalations and exhalations also pass over an oxygen sensor providing an output as a function of the instantaneous oxygen concentration in the flowing gas. These two signals are provided to a computer which integrates them to derive signals representative of the inhaled and exhaled oxygen volume. From these measurements the oxygen consumption, respiratory quotient and related respiratory parameters are calculated and displayed.
One advantage of the present invention is that the indirect calorimeter utilizes an ultrasonic transit time flow meter and a fluorescence quench oxygen sensor. Both of these sensors operate upon the respiratory gasses as they pass through a flow tube with a substantially continuous, uninterrupted internal diameter so that the flow is substantially laminar. Previous indirect calorimeters, including those disclosed in the above-described U.S. patents, have employed flow measurement techniques that require protrusions in the flow path such as pressure differential transducers, hot wire transducers or the like. Great difficulties are encountered in maintaining a largely laminar flow in transducers of this type, resulting in inaccuracies in the flow measurement. The present invention preferably employs a volume flow meter which transmits ultrasonic pulses through the flow stream in a direction either parallel to the flow path or at least having a component parallel to the flow path. The transit time of the pulses is a function of the flow rate of the gas and because the interior diameter of the flow tube wall is substantially uninterrupted, laminar flow conditions are maintained providing a high uniformity of measurement.
Another advantage of the present invention is that an indirect calorimeter is provided that directly measures the oxygen concentration in the inhaled and exhaled gasses passing through the flow tube by a technique which does not introduce any protuberances into the flow area and which may be positioned to measure the oxygen content in the same area in which flow is measured. Thus, unlike previous systems which require some linear separation between the point of flow measurement and the point of gas analysis, and accordingly would result in inaccuracies were the two to be integrated, the present system does not create any phase lag between the oxygen measurement and the flow measurement which would otherwise result in inaccuracies and the need for signal processing to correct for the displacement of the measurements. The preferred embodiment of the invention employs a fluorescence quench technique for oxygen measurement which utilizes a fluoresceable chemical disposed on the interior diameter of the flow wall in the area of ultrasonic pulse transmission. This fluorescent coating may be formed on the tube wall directly or supported on the end of a fiber optic probe terminating in alignment with the interior diameter of the tube. This coating is subjected to exciting radiation from the exterior of the tube and the resulting fluorescence may be measured from the exterior. The fluorescence is quenched by oxygen passing over the coating and the percentage of oxygen in the flow tube can be instantaneously measured by the intensity of the fluorescence.
Still another advantage of the present invention is that the flow tube is preferably formed as a disposable insert which may be inserted into a permanent, reusable structure which includes the ultrasonic transmitter and receiver and the fluorescence oxygen sensor. The fluorescent coating may be covered on the tube side with a microbial filter formed as part of the disposable insert. This filter prevents the fluorescent coating from being bacterially contaminated. The disposable insert is utilized to avoid the spread of disease from user to user in situations in which the indirect calorimeter is used by a succession of persons. The insert is preferably produced of an inexpensive material such as plastic.
A further advantage of the present invention is that the disposable insert is supported by a disposable breathing mask that covers the nose and the mouth of the user, allowing normal breathing over the measurement time. Most prior art devices have employed mouthpieces; however, it has been determined that in many users the mouthpiece can induce a mild form of hyperventilation which increases the user""s energy consumption and results in erroneous metabolic readings. In one embodiment of the present invention, the metabolic measurement components are integrated with and are contained within the mask with no requirement for external connections. When the mask is attached to the user""s head by straps, adhesive, or the like, it allows a full range of user movement during the measurement. Thus, it can be used during normal exercise to allow determination of the effect of that activity on respiratory parameters and may also be used to measure resting energy expenditure. The increased user comfort resulting from the elimination of connections between the mask and associated apparatus allows measurements to be made over longer periods of time and minimizes the labored breathing often associated with conventional respiratory masks which affects accurate measurement of energy expenditure.
Still a further advantage of the present invention is that the mask preferably incorporates a nasal spreader on its interior surface which adhesively attaches to the nares of the user""s nose and pulls them outwardly to enlarge the nose flow area and minimize the energy expenditure in breathing, which is often increased with conventional masks.
Yet still a further advantage of the present invention is that the computation unit and display and controls are supported in a separate desktop unit and connected to the sensors within the mask by highly flexible cables.
Other features and advantages of the present invention will be readily appreciated, as the same becomes better understood after reading the subsequent description taken in conjunction with the accompanying drawings.